It is well known in the art to remove acid gas (H2S and CO2) and other contaminants from gas streams using more or less selective solvents. However, as many gas streams also contain substantial quantities of olefins, heavy hydrocarbons, sulfur dioxide, and especially mercaptans and other organic sulfurous compounds (e.g. methyl mercaptan, ethyl mercaptan, butyl mercaptan, carbonyl sulfide, dimethyl disulfide, carbon disulfide, propanethiol, and thiophene), undesirable side reactions with the solvent or other treating medium (e.g., fixed bed catalyst) often render the solvent/treating and reactor media ineffective and necessitate plant shutdown. To avoid difficulties associated with such side reactions, pre- and post treatment units can be implemented to render a gas stream more suitable processing and/or for emission into the environment.
An exemplary known gas treatment configuration that employs a solvent is depicted in Prior Art FIG. 1 in which the acid gases are absorbed in an absorber 50 that forms a rich solvent 3. The rich solvent 3 is then flashed in a flash drum 52, with the vapors being recycled to the absorber 50 while the liquid 6 is routed to the regenerator 58. Here, the acid gases are removed from the solvent to form the lean solvent that is cross exchanged with the rich solvent 6 before entering again the absorber 50. The so absorbed acid gases and other sulfurous compounds are stripped in the regenerator and separated in an overhead separator 70 to form a reflux 12 for the regenerator, a contaminant vapor 10, and a contaminant liquid 11. Contaminant gases 10 are typically further processed in a Claus plant 72, while the contaminant liquids 11 are frequently recycled to a refinery for disposal. Most commonly, the solvent is a physical solvent or amine (e.g. propylene carbonate, tributyl phosphate, methyl pyrolidone, and other various polyethylene glycol dialkyl ethers, formulated tertiary amine or other amines) that can be used to at least some degree in the removal of mercaptans and heavy hydrocarbons.
While such processes generally operate satisfactorily under certain circumstances, several problems nevertheless remain. Among other things, amine solvents are often ineffective in removal of mercaptans and organic sulfur. Physical solvents can typically absorb these contaminants. However, such solvents tend to co-absorb hydrocarbons and thus produce laden liquid and vapor waste streams that create emission problems downstream. Additionally, residual olefinic hydrocarbons in the treated gas may further react with fixed bed absorbents or hydrotreating catalysts forming gums rendering such processes unsafe and inoperable. Still further, high levels of heavier hydrocarbons and mercaptans in the acid gases often create conversion problems in a downstream sulfur plant. For example, for complete destruction of mercaptans and other organic sulfurs contaminants, the Claus reaction furnace must be operated at a high flame temperature, which will significantly reduce the life of the sulfur plant. Moreover, even with higher flame temperatures, destruction of the heavier mercaptans is often difficult and incomplete, which results in fouling of the reaction catalysts and ultimately plant shutdown
To circumvent at least some of the problems associated with inadequate contaminant removal, various pre- and post treatment methods have been employed. Unfortunately, most of such methods tend to be relatively ineffective, inefficient and costly, and where contaminants are removed by a fixed bed absorbent process, they may further pose a disposal problem for the spent absorbent. Therefore, various problems associated with operating efficiency, effluents, emissions, and product qualities, and particularly in the downstream sulfur plant, tail gas unit and fuel gas conditioning unit still remain. For example, acid gas produced from such treating processes is generally poor in quality (e.g., comprising significant quantity of contaminants, and/or a relatively large quantity of co-absorbed CO2 and hydrocarbons), and the treated gas typically contains significant quantities of environmentally undesirable olefinic hydrocarbons, which often requires additional processing and energy consumption, thereby increasing the capital and operating costs.
In still other known processes, especially with hydrotreater processing high olefinic hydrocarbons (ethylene, propylene, propyldienes, butenes and butadienes and heavy olefins), the reliability and available of such units are typically very low due to fouling from the heavier components (C6+) in the feed gases. The residual olefinic hydrocarbons in the treated gas from these processes will result in excessive NOx formation in the burners and power generation equipment that may necessitate shutdown of the facility. Therefore, while various gas processing treatments and configurations are known in the art, all or almost all of them suffer from one or more disadvantages, and especially where the feed gas comprises relatively high levels of acid gases, olefinic hydrocarbons, mercaptans and organic sulfurs contaminants.